Conventional helicopter windscreen panels are of a laminated structure comprising a glass outer sheet which is bonded by a polyvinylbutyral interlayer to an inner sheet of plastics material, usually a stretched acrylic sheet.
There is a requirement for rapid replacement of windscreen panels, in particular if they are damaged under combat conditions, and the invention is concerned with an improvement in the use of an easily softened thermoplastic material which can be used to adhere a window panel to a window frame when a window panel is being fitted, and can also be resoftened for rapid removal of a damaged window panel.
It has been proposed in GB No. 1 207 483 to use an attachment strip of thermoplastic adhesive around the inner edge of an automobile windscreen, which strip embodies an electrical resistance wire. When the wire is connected to an electrical supply it heats and the thermoplastic material of the attachment strip softens so that the glass windscreen can then be adhered to the vehicle body. The thermoplastic strip used is a butyl rubber strip which is opaque.
GB No. 1 210 288 proposes the use of a tacky strip of uncured Neoprene-based material in which there is a groove in which a resistance wire runs and which is filled with a polysulphide mastic material which is softer than the material of the strip and which protrudes from the groove. When current is passed to soften the strip the mastic substance enters into any irregularities in the flange to which the window panel is being adhered.
The main problem when attempting to use a technique of this kind in the glazing of an aircraft windscreen is that a considerable amount of the adhesive strip would remain adhered to the window frame when the strip is heated for removal of a damaged window panel. This would mean that the window frame would have to be scraped and cleaned before a new window panel could be fitted.
It is a main object of the invention to overcome this problem.